1. 1 COMP 206: Computer Architecture and Implementation Montek Singh Mon., Sep 30, 2002 Topic: Instruction-Level Parallelism (Dynamic Scheduling: Tomasulo’s Algorithm)

2. 2 Dynamic Scheduling: Tomasulo’s Algorithm  For IBM 360/91 (about three years after CDC 6600)  Goal: High performance without special compilers  Differences between IBM 360 and CDC 6600 ISA IBM has only 2 register specifiers/instruction versus 3 in CDC 6600 IBM has only 2 register specifiers/instruction versus 3 in CDC 6600 IBM has 4 FP registers versus 8 in CDC 6600 IBM has 4 FP registers versus 8 in CDC 6600  Differences between Tomasulo Algorithm and Scoreboard Control and buffers distributed with Function Units versus centralized in scoreboard; called “reservation stations” Control and buffers distributed with Function Units versus centralized in scoreboard; called “reservation stations” Registers in instructions replaced by pointers to reservation station buffer Registers in instructions replaced by pointers to reservation station buffer Hardware renaming of registers to avoid WAR and WAW hazards Hardware renaming of registers to avoid WAR and WAW hazards Common Data Bus broadcasts results to all FUs (forwarding) Common Data Bus broadcasts results to all FUs (forwarding) Load and Stores treated as FUs as well Load and Stores treated as FUs as well

3. 3 Tomasulo: Organization 1 2 3 4 5 6 Floating point operand stack Decoder FP registers (4) Tags Busy bit TagDataTagData TagDataTagData TagDataTagData Adder (2 cycle, pipelined) TagDataTagData TagDataTagData Multiply (3 cycle, pipelined) Divide (12 cycle, not pipelined) 10 11 12 8989 From memory To memory From I-Fetch Status To all tags Common data bus (CDB) TagAddr TagAddr TagAddr Data

4. 4 More Details of Tomasulo Organization  Entities that produce values are assigned 4-bit tags 1, 2, 3, 4, 5, 6 for load buffers 1, 2, 3, 4, 5, 6 for load buffers 8, 9 for multiplier reservation stations 8, 9 for multiplier reservation stations 10, 11, 12 for adder reservation stations 10, 11, 12 for adder reservation stations Tag 0 indicates presence of valid data Tag 0 indicates presence of valid data  FP registers have “busy bits” 0 means that register holds valid data 0 means that register holds valid data 1 means that it is waiting to receive value from source identified by its tag field 1 means that it is waiting to receive value from source identified by its tag field

5. 5 Tomasulo: Representing Data Dependences  Inputs Operand is a register with busy bit = 0 Operand is a register with busy bit = 0  Data copied immediately (through register bus) into reservation station  Tag field of RS set to 0 Operand is a register with busy bit = 1 Operand is a register with busy bit = 1  Tag field of RS receives a copy of the register tag field Operand is a load buffer that contains valid data Operand is a load buffer that contains valid data  Data copied into RS Operand is a load buffer that is awaiting data Operand is a load buffer that is awaiting data  Tag field of RS receives tag of load buffer  Outputs Output is a register Output is a register  Busy bit set to 1, tag set to RS tag Output is a store buffer Output is a store buffer  Tag set to RS tag, destination address set

6. 6 Three Stages of Tomasulo Algorithm 1. Issue: get instruction from FP operation queue If reservation station free, the scoreboard issues instruction and sends operands (renames registers) If reservation station free, the scoreboard issues instruction and sends operands (renames registers) 2. Execution: operate on operands (EX) When both operands ready then execute; if not ready, watch CDB for result When both operands ready then execute; if not ready, watch CDB for result 3. Write Result: finish execution (WB) Write on Common Data Bus to all awaiting units; mark reservation station available Write on Common Data Bus to all awaiting units; mark reservation station available

7. 7 Tomasulo: State Transitions In case of a CDB conflict, earlier instruction has priority If more than one instruction is enabled in the reservation stations of adder or multiplier in same cycle, top entry has priority If CDB transfer and issue occur in same cycle, CDB transfer is assumed to occur first Every instruction should spend at least one cycle in R stage If an instruction being issued both reads and writes the same register, and the source operand is actually in the register (busy bit = 0), then first the register is read, and then its busy bit is turned to 1, making it unreadable Load Buffer Register

8. 8 Tomasulo: Example 100: F0  A 101: F0  F0 + F1 102: F0  F0 + B 103: F2  F2 + F3 104: F1  F1 * F2 105: C  F1 106: F0  F1 / F0 100: F0  A 101: F0  F0 + F1 102: F0  F0 + B 103: F2  F2 + F3 104: F1  F1 * F2 105: C  F1 106: F0  F1 / F0 100 101 102 106 103 104 105 IIssued RIn reservation station XIn execution WWriting result through CDB

9. 9 Tomasulo Example Cycle 0  System is quiescent

10. 10 Tomasulo Example Cycle 1  (A) will arrive at tag 4  (F0) will come from tag 4  F0 is set to “busy”

11. 11 Tomasulo Example Cycle 2  (F0) will be produced at tag 10  Right input of adder came from register (tag bit = 0)  Left input of adder will come from tag 4  Forwarding tag of F0 has been changed from 4 to 10

12. 12 Tomasulo Example Cycle 3  (F0) will be produced at tag 11  (B) will arrive at tag 3  Right input of adder will come from tag 3  Left input of adder will come from tag 10  (A) arrives from memory  Forwarding tag of F0 has been changed from 10 to 11

13. 13 Tomasulo Example Cycle 4  (F2) will be produced at tag 12  Right input of adder came from register (tag bit = 0)  Left input of adder came from register (tag bit = 0)  (A) with tag 4 is broadcast on CDB  Adder (at tag 10) picks it up, and is thereby enabled  The instruction that will write F2 has already read the old contents of F2

14. 14 Tomasulo Example Cycle 5  (F1) will be produced at tag 8  Right input of multiplier will come from tag 12  Left input of multiplier came from register (tag bit = 0)  Adder (at tag 10) starts computing  (B) arrives from memory

15. 15 Tomasulo Example Cycle 6  Memory address of destination is C  Data will come from tag 8  Adder (at tag 10) finishes computing  (B) with tag 3 is broadcast on CDB  Adder (at tag 11) picks it up  Adder (at tag 12) starts computing

16. 16 Tomasulo Example Cycle 7  (F1) will be produced at tag 9  Right input of divider will come from tag 11  Left input of divider will come from tag 8  Result of adder (with tag 10) is broadcast on CDB  Adder (at tag 11) picks it up and is thereby enabled  Adder (at tag 12) finishes computing

17. 17 Tomasulo Example Cycle 8  Result of adder (at tag 12) is broadcast on CDB  Multiplier (at tag 12) picks it up and is thereby enabled

18. 18 Tomasulo Example Cycle 9  Multiplier (at tag 8) starts computing  Adder (at tag 11) finishes computing

19. 19 Tomasulo Example Cycle 10  Result of adder (with tag 11) is broadcast on CDB  Divider (at tag 9) picks it up  Register F0 picks it up

20. 20 Tomasulo Example Cycle 11  Multiplier (at tag 8) finishes computing

21. 21 Tomasulo Example Cycle 12  Result of multiplier (at tag 8) is broadcast on CDB  Divider (at tag 9) picks it up, and is thereby enabled  Store buffer (at tag 1) picks it up, and is thereby enabled

22. 22 Observations on Tomasulo’s Algorithm  Instructions: move from decoder to reservation stations in program order in program order dependences can be correctly recorded dependences can be correctly recorded  Data Flow Graph: The graph of pointers connecting the RS, registers, and memory buffers helps accomplish out-of-order sequencing of instructions helps accomplish out-of-order sequencing of instructions  Chief cost of this scheme: high-speed associative hardware RS hardware has to search for tags when CDB broadcasts some value with its tag RS hardware has to search for tags when CDB broadcasts some value with its tag  Full load bypassing is supported load and store buffers are treated just like functional units load and store buffers are treated just like functional units additional hardware on 360/91 also supported load forwarding additional hardware on 360/91 also supported load forwarding

23. 23 Tomasulo: Example of Load Bypassing 200: F0  A 201: F0  F0 / F1 202: C  F0 203: F0  D 204: F0  F0 * F2 200: F0  A 201: F0  F0 / F1 202: C  F0 203: F0  D 204: F0  F0 * F2  Instruction 202 depends on instructions 200 and 201, so instruction 203 will start executing much before 202 (assuming C and D are found to be different memory addresses)  Work out details off-line

24. 24 Tomasulo: “Loop Unrolling in Hardware”  360/91 supported limited kind of speculation Small loops could be held in a loop buffer Small loops could be held in a loop buffer Loop closing branches were predicted as taken Loop closing branches were predicted as taken  This has the effect of loop unrolling at run-time Given the small number of FP registers in machine, software loop unrolling was not a viable option Given the small number of FP registers in machine, software loop unrolling was not a viable option

25. 25 Tomasulo Loop Example Loop: L.DF00R1 MULT.DF4F0F2 S.DF40R1 SUBIR1R1#8 BNEZR1Loop  Multiply takes 4 clocks  Loads have cache misses

26. 26 Loop Example Cycle 0

27. 27 Loop Example Cycle 1

28. 28 Loop Example Cycle 2

29. 29 Loop Example Cycle 3

30. 30 Loop Example Cycle 4

31. 31 Loop Example Cycle 5

32. 32 Loop Example Cycle 6 Load2

33. 33 Loop Example Cycle 7

34. 34 Loop Example Cycle 8

35. 35 Loop Example Cycle 9

36. 36 Loop Example Cycle 10

37. 37 Loop Example Cycle 11

38. 38 Loop Example Cycle 12

39. 39 Loop Example Cycle 13

40. 40 Loop Example Cycle 14

41. 41 Loop Example Cycle 15

42. 42 Loop Example Cycle 16

43. 43 Loop Example Cycle 17

44. 44 Loop Example Cycle 18

45. 45 Loop Example Cycle 19

46. 46 Loop Example Cycle 20

47. 47 Loop Example Cycle 21

48. 48 Summary of Tomasulo’s Algorithm  Prevents registers as bottleneck  Avoids WAR and WAW hazards of scoreboard  Allows loop unrolling in hardware  Not limited to basic blocks (provided we have branch prediction)  Lasting contributions Dynamic scheduling Dynamic scheduling Register renaming Register renaming Load/store disambiguation Load/store disambiguation  Next: Dynamic branch prediction